In a direct sequence (DS) CDMA transmission, information data is transmitted after it undergoes a primary modulation, followed by a secondary modulation which spreads the primary modulated signal with a high rate spreading signal. At a receiving side, the wideband received signal (spread signal) is first converted into the original narrow-band signal through a process called despreading, and then undergoes a usual demodulation. The despreading at the receiving side is carried out by detecting correlation between the received signal and a replica of the spreading code synchronized in phase with the spreading code in the received signal. Accordingly, it is necessary for recovering the information data to synchronize the spreading code replica with the spreading code in the received signal.
The synchronization roughly falls into two processes: acquisition and tracking. The acquisition captures the phase difference between the received spreading code and the spreading code replica within a range sufficiently narrower than .+-. one chip because the autocorrelation of a quadrature code used as the spreading code can usually be obtained only within .+-. one chip. Then, the tracking keeps the phase difference between the two codes within this range. Since the present invention relates only to the acquisition, a conventional acquisition method will be described below.
The acquisition of the spreading code is generally carried out as follows: First, the correlation between the received signal and the spreading code replica is taken by multiplying the two codes, and by integrating the product for one spreading code period. Next, the establishment of the acquisition is decided by square-law detecting the correlation output followed by deciding whether or not the detection output exceeds a threshold. Since the phase of the carrier is not synchronized between the transmission side and the reception side, it is difficult to know the phase of the carrier before despreading. Thus, the non coherent method is used in which the effect of the carrier phase is removed by square-law detecting the correlated and integrated signal before performing the threshold decision.
The correlation of the acquisition can usually be detected using a sliding correlator carrying out time integral, or a matched filter performing space integral. The matched filter is an FIR filter (transversal filer) having the tap number corresponding to the processing gain and using the spreading code as tap coefficients. Since the matched filter achieves the correlation detection between the spreading code in the received signal and the spreading code replica at once by using the space integral, the acquisition is achieved in a fraction of time. However, the matched filter has taps whose number corresponds to the processing gain. In addition, the correlation detection must be performed at a rate at least twice the chip rate because it is necessary for the matched filter to adjust its sampling timing at a peak position with the band-limited received signal. Thus, the matched filter requires the tap number twice the processing gain, which presents a shortage that its circuit scale becomes larger than that of the sliding correlator.
The present invention relates to the acquisition of a long code with a very long period as compared with an information symbol, and hence it is considered very difficult to realize by using the matched filter. Thus, the acquisition with the sliding correlator will be considered in connection with the present invention.
The correlation is calculated between the spreading code replica and the received signal including noise, and the correlation output undergoes an envelope detection through a square-law detector. The detected output is integrated for a dwell time .tau. in an integral & dump circuit. The integrated output undergoes threshold processing. Although the sliding correlation method is simple in circuit configuration, it has a shortage that it takes a long time for the acquisition.
In the CDMA applied to a cellular, control information is exchanged between a base station and a mobile station through a control channel before a traffic channel is established for transmitting information data represented by voice. Generally speaking, an increasing period of spreading code will provide a greater number of the spreading codes, thereby increasing the number of multiplexing, although a time taken for the acquisition grows longer because of an increase in phase uncertainty. Taking account of this characteristic, a method is proposed in which a short code is used for a control channel and a long code is applied to a traffic channel by superimposing it over the short code, the short code having a period equal to one symbol interval of the information data, and the long code having a much longer period than the information data symbol. In this method, the acquisition of the control channel is achieve using the short code, and that of the traffic channel is carried out by inserting phase information on the starting position of the long code in the control channel. Thus, the acquisition of the traffic channel is started from a state in which the chip phase is nearly synchronized between the base station and the mobile station. As a result, the acquisition of both channels can be established in a short time.
This method, however, presents a problem in that it employs a short code in the control channel. Spreading the control channel with the short code will substantially restrict the number of the control channel because the number of the quadrature codes is determined by the code length. When assigning the limited number of the control channels, spreading code management is required in connection with the control channel. To avoid such management, an increasing number of control channel is required, in which case, it is necessary to increase the code length of the control channels to some extent. Thus, it is also required for the control channel to use spreading codes with a period longer than one symbol period of the information data. In this case, speed up of the acquisition becomes important.